1. Field of the Invention
The present invention relates to composition analysis, and more specifically, it relate) to multi-dimensional composition analysis of objects using sequences of ultrashort laser pulses to ablatively present trains of small mass-packets for examination by a variety of analytic instruments.
2. Description of Related Art
The surface region of a sample may be analyzed by removing surface material and analyzing that removed material by mass spectrometry. The step of removing sample material may be accomplished in a number of ways generally involving the input of heat, the transfer of momentum or an electronic excitation. For example, in a laser microprobe an intense focused laser probe ablates sample material and provides ions for analysis, as reviewed by R. J. Cotter and J. C. Tabet in American Laboratory 1984, 16(4), pages 86 to 99. Laser probes of low power density are particularly advantageous in the analysis of bio-organic molecules as reported by M. A. Posthumus et al in Analytical Chemistry 1978, 50(7), pages 985 to 991. Other widely used techniques include secondary ion mass spectrometry (SIMS) and fast atom bombardment (FAB) mass spectrometry wherein a beam of ions or atoms sputters material from a sample, as reviewed by A. Benninghoven et al. in SIMS, volume 86 of Chemical Analysis, 1987 published by John Wiley and Sons.
Alternatively, electrons or photons may be employed for stimulating desorption from adsorbed layers or from the outermost atomic layers of a sample as described by J. A. Kelber and M. L. Knotek in the Springer Series in Surface Science, 1985, volume 4, pages 182 to 187. Thus there is available a range of techniques for removing and subsequently analyzing material from a range of depths, from the outer monolayer down to several tens or hundreds of microns. Moreover these techniques may be combined with an eroding technique such as ion milling to investigate essentially any desired depth.
The above-mentioned techniques generally produce more neutral particles than ions and that neutral emission is not subject to discriminating influences that can make ionic emission unrepresentative of the surface composition. It has long been recognized that it would be advantageous to provide means for post-ionizing the neutrals to facilitate their analysis by mass spectrometry, and suitable techniques have been reviewed by W. Reuter in the Springer Series in Chemical Physics 1986, 44 pages 94 to 102 and by A. Benninghoven et al (op cit 1987) pages 937 to 949. Post-ionization by an electron beam is described by R. E. Honig in the Journal of Applied Physics 1958, 29(3), pages 549 to 555; by A. J. Smith in the Journal of Applied Physics 1963, 34, pages 2489 to 2490; by D. Lipinsky et al in the Journal of Vacuum Science and Technology, 1985, A3, pages 2007 to 2017; and by I. R. M. Wardell in U.S. Pat. No. 3,660,655. A disadvantage of electron beam post-ionization is that it provides a low ion yield, given by W. Reuter as 10xe2x88x929 ions per atom. Post-ionization by an electron gas or by a plasma has also been reported with ion yields according to W. Reuter of 10xe2x88x929 and 10xe2x88x927 respectively, whereas more satisfactory yields in the region of 10xe2x88x922 to 10xe2x88x924 are reported for by various laser beam post-ionization techniques. Prior to this laser work, photoionization by light from spark or other discharge lamps had been employed in the analysis of gaseous and thermally evaporable samples as reported for example by W. Genuit and J. J. Boon in the Journal of Analytical and Applied Pyrolysis 1985, 8, pages 25 to 40; by M. E. Akopyan et al in Instrum Exp Tech 1972, 15(2), pages 1481 to 1482; in U.S. Pat. Nos. 4,521,054, 4,028,617 and 4,476,392; and as reviewed by N. W. Reid in the International Journal of Mass Spectrometry and Ion Physics 1971, 6, pages 1 to 31. Such photoionization mass spectrometry is generally compared unfavorably with electron impact ionization mass spectrometry because of its low ion yield, as described by W. Poschenrieder and P. Warneck in the Journal of Applied Physics 1966, 37(7), pages 2812 to 2820. D. F. Hunt in The International Journal of Mass Spectrometry and Ion Physics 1982, 45, pages 111 to 123 points out that lasers are required to provide a sufficiently high photon flux as exemplified by the work of M. Seaver et al in the International Journal of Mass Spectrometry and Ion Physics 1980, 34, pages 159 to 173 and reviewed by R. J. Cotter in Analytica Chimica Acta 1987, 195, pages 45 to 59.
Techniques for laser post-ionization of sputtered neutrals are generally categorized as using either resonant or non-resonant ionization. Resonant ionization occurs when the laser frequency is such that its associated photon energy matches the energy required to induce at least one electronic transition in the ionizing process. Several suitable resonance schemes are described by J. E. Parks et al in Thin Solid Films 1983, 108(2), pages 69 to 78, and the technique has been described variously by D. W. Beekman et al. in the International Journal of Mass Spectrometry and Ion Physics 1980, 34, pages 89 to 97; by N. Winograd et al in Chemical Physics Letters 1982, 88(6), pages 581 to 584, and in U.S. Pat. No. 4,442,354. In this technique the ionizing laser is tuned to correspond to a resonant transition and thus produces enhanced ionization with high selectivity of the ionized species in the presence of other substances for which the resonance condition is not satisfied. Such selectivity can be advantageous but requires some knowledge of the composition of a sample in advance of the analysis. By contrast, a technique based on non-resonant ionization as reported by C. H. Becker et al in U.S. Pat. No. 4,733,073 is inherently non-selective.
In Analytical Chemistry 1984, 56, pages 1671 to 1674, C. H. Becker et al give the major requirement of their non-resonant ionization technique as being a laser intensity high enough to achieve significant ionization probabilities. Non-resonant multi-photon ionization proceeds by a series of transitions to one or more virtual states which are not true eigenstates of the atom but between which transitions are possible in a very high light intensity as described by N. B. Delone in Soviet Physics Usp 1975, 18(3), pages 169 to 189. C. H. Becker et al have reported single photoionization studies of the surfaces of bulk polymers, and of molecular adsorbates, respectively in the Journal of Vacuum Science and Technology A 6(3), 1988, pages 936 to 940 and the Journal of the American Chemical Society 1988, 110, pages 2323 to 2324. Arrangements for the laser postionization of sputtered neutrals have also been reported in PCT Patent Applications Nos. W087/07762 and W088/06060 covering both resonant and non-resonant ionization processes.
A pervasive need exists to better understand the isotopic and elemental composition of complex (e.g., 3-D) macroscopic objects (ranging from Martian meteorites and fossilized dinosaur skeletons to epitaxially-grown novel materials and internationally-derived objects of uncertain but suspected-interesting origins or histories) on spatial scales of micrometers and within end-to-end object-analysis time-scales of minutes to days. The present invention provides a generally applicable solution to this analytic challenge.
It is an object of the present invention to provide a method and apparatus for sequentially disassembling an object of arbitrary shape, size and composition into a train of (usually very small) mass-packets and analyzing these mass-packets as they are removed from known locations on the parent-object for their chemical-elemental, molecular (fragment), or mass-isotopic composition.
Mass-packet removal from a parent-object is accomplished by exceedingly rapid ablation with a train of femtosecond-duration focused laser pulses, which heat the corresponding portion of the object""s surface to temperatures far in excess of vaporization temperatures on time-scales very small compared to thermal conduction, thermal radiation or hydrodynamic relaxation processes. The superheated mass-packets that then xe2x80x9cjump offxe2x80x9d of the surface are then readily analyzed by standard means for their composition.
The present materials-analysis invention is referred to herein as xe2x80x9cComposition Analysis By Scanning Femtosecond Laser Ultraprobingxe2x80x9d (CASFLU), and employs exceedingly short-duration laser pulses (txc2xdxe2x89xa610xe2x88x9212 seconds) of exceptionally high intensity (Ixe2x89xa71013 W/cm2) to heat a well-defined parcel of surface material to a large multiple of its vaporization temperature, raising the parcel into the temperature regime where it is relatively highly ionized (i.e., Txe2x89xa76,000 K, or kTxe2x89xa70.5 eV, where k is Boltzmann""s constant), well before this material can expand-and-cool hydrodynamically or diffuse its heat into surrounding portions of the object being analyzed. These laser pulse-heating time-scales are typically 1,000 times shorter than the corresponding hydrodynamic times, and are even far shorter (by more than 100-fold) than the associated thermal conduction times for the imposed heat to be diffusively transported into surrounding material. Their use creates a sequence of (usually) tiny mass-parcels of superheated vapor at solid density, which then xe2x80x9cjump offxe2x80x9d the surface so irradiated in succession, each one leaving the surrounding surface essentially unperturbed, e.g., available for subsequent inspection, analysis, etc., without major perturbation or xe2x80x9cpre-processingxe2x80x9d by thermal or mechanical effects of the mass-removal.
This exceptionally xe2x80x9ccleanxe2x80x9d, side effects-free removal of mass-packets, which is unique to CASFLU relative to all the analytic approaches surveyed in the foregoing, permits the systematic, sequential disassembly of surfaces and volumes of objects of complex structure and highly-variable composition by repetitive, spatially-precise extraction of such mass-packets from exposed surfaces. Each such mass-parcel then may be readily composition-analyzed very soon after it commences to xe2x80x9cjump offxe2x80x9d the underlying surface, e.g., by a variety of spectrometric means of which (near-)optical- and mass-spectroscopy are major examples. Indeed, bringing nearly the entire mass-parcel to an electron temperature xe2x89xa70.5 eV first-ionizes a substantial fraction ( greater than 1%) of all materials, i.e., gives an ion-to-atom fraction  greater than 10xe2x88x922, so that subsequent spectroscopy in the vibrational and electronic excitation spectra and in the mass spectrum usually doesn""t require post-ionization, and thus is substantially facilitated.